Process for preparing nonconductive substrates

ABSTRACT

The process of electroplating a metal layer to the surface of a nonconductive material which comprised the steps: 
     (a) contacting said substrate surface with an alkaline permanganate solution for an effective time and at an effective concentration and at an elevated temperature to prepare said substrate surface for a metal layer to be electroplated thereto; 
     (b) then contacting said substrate surface with an aqueous neutralizer/conditioner solution, said solution comprising water, at least one neutral or acidic reducing agent, and at least one polyelectrolyte polymer conditioner; 
     (c) then contacting said substrate with a liquid dispersion of carbon black comprised of: 
     (1) carbon black particles having an average particle size of less than about 3 microns in said dispersion; 
     (2) an effective dispersing amount of a surfactant which is compatible with said carbon black; and 
     (3) a liquid dispersing medium, wherein the amount of carbon black is sufficient to coat substantially all of the nonconducting substrate surface and is less than about 4% by weight of said liquid dispersion; 
     (d) separating substantially all of the liquid dispersing medium from said applied dispersion, thereby depositing said carbon black particles in a substantially continuous layer on said nonconductive substrate surface; and 
     (e) electroplating a substantially continuous metal layer over the deposited carbon black layer on said nonconduting substrate surface. 
     A neutralizer/conditioner solution comprising a mixture of water, at least one neutral or acidic reducing agent, and at least one polyelectrolyte polymer conditioner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for preparing a nonconductivesubstrate for electroplating. Further, this invention relates to animproved process for preparing the through hole walls of a printedwiring board (PWB) for electroplating. Still further, this inventionrelates to an aqueous neutralizer/conditioner solution.

2. Brief Description of Prior Art

For the past thirty years the printed wiring board industry has reliedon the electroless copper deposition process to prepare through holewalls in printed wiring boards for electroplating. These plated throughhole walls are necessary to achieve connections between two metalcircuit patterns on each side of a printed wiring board or, in additionto this, between the inner layer circuit patterns of a multilayer board.

The electroless deposition of copper onto the through hole wallstypically consists of precleaning a PWB and then processing according tothe following sequence of steps:

Step 1. Preactivator

Step 2. Pd/Sn Activator

Step 3. Rinse

Step 4. Accelerator

Step 5. Rinse

Step 6. Electroless Copper Deposition

Step 7. Electroplating

These processed boards may also be photoimaged before the electroplatingprocess. Typically, the deposited copper layer on each through hole wallis about 1±0.2 mil thick.

Conventional electroless processes have several commercialdisadvantages. They require a relatively long process time. The multipletreatment baths have complex chemistry which may require constantmonitoring and individual ingredients which may require separatereplenishment. The palladium/tin activator also may require expensivewaste treatment. Furthermore, these electroless process baths may bevery sensitive to contamination. Finally, the multiplicity of rinsebaths may require large amounts of water.

Prior to the electroless method of plating through holes, graphite wasemployed to prepare the walls of the through holes for plating. Forexample, U.S. Pat. No. 3,099,608, which issued to Radovsky et al. onJuly 30, 1963, teaches a process for preparing the through hole walls ofprinted circuit boards for electroplating by initially depositing insaid through holes a thin electrically nonconductive film of palladiummetal in at least a semi-colloidal form. The patent discloses thatgraphite had been used previously as a conductive layer forelectroplating thereon. See column 1, lines 63-70 and column 4, line 72to column 5, line 11. These patentees noted several deficiencies withthat graphite process including lack of control of the graphiteapplication, poor deposit of the resultant electroplated metal,nonuniform through hole diameters, and low electrical resistance of thegraphite.

U.S. Pat. No. 3,163,588, which issued to Shortt et al. on Dec. 29, 1964,also mentions that graphite or its equivalents may be employed to renderthrough hole walls of electric circuit boards conductive for laterelectroplating metals thereon. See column 3, line 45 to column 4, line2.

U.S. Pat. No. 4,581,301, which issued to Michaelson on Apr. 8, 1986,teaches the application of a seed layer of conductive particles, such as"carbon", on the walls of through holes before electrolytically platingcopper over the seed layer. This reference does not explicitly teach theuse of a continuous layer of carbon black dispersion in the seed layer,and does not recognize the advantage of using very small particles ofcarbon black such as presently claimed. See column 7, lines 63-66 whichrefer to particles passing through a 400 mesh screen. A 400 mesh screenis equivalent to about 35 microns.

Separately, graphite has been employed in numerous processes forpreparing a nonconducting material for a metal coating or plating. Forexample, U.S. Pat. No. 409,096, which issued to Alois Blank on Aug. 13,1889, teaches a process for applying copper to asbestos roofing materialwhich comprises first applying powdered plumbago (graphite) in avolatile liquid such as varnish to the surface of the asbestos, thenevaporating the volatile liquid to coat the asbestos fibers with fineparticles of plumbago. The plumbago coated asbestos sheets are thenimmersed in a copper electroplating solution and electric current isapplied to the coated asbestos sheet to form a thin film of copperthereon. The copper coated sheet is then immersed in a bath of moltenmetal such as tin, lead, or zinc, and is then removed from the moltenbath to effect solidification of the molten metal. The resulting metalcoated asbestos sheet is described as being relatively flexible, anonconductor of heat and substantially fireproof.

U.S. Pat. No. 1,037,469, which issued to Goldberg on Sept. 3, 1912, andU.S. Pat. No. 1,352,331, which issued to Unno on Sept. 7, 1920, discloseprocesses for electroplating nonconducting materials by first coatingthe nonconducting material with wax, then coating the wax with a slurryof finely divided particles of graphite or other metal, followed byelectroplating of the dust-coated surface with copper or other metal.Neither of these processes are particularly suitable for use in coatingthe hole walls of circuit boards because the holes are normallyextremely narrow in diameter and immersing in wax would tend to plug thehole and prevent coating the hole walls with an electroplating material.

U.S. Pat. No. 2,243,429, which issued to Laux on May 27, 1941, disclosesa process for electroplating a nonconductive surface by "graphiting" athin layer onto the nonconducting surface followed by applying a copperlayer electrolytically and "finally a further electrolytic deposit ofanother metal" is placed thereon.

Separately, carbon black formulations have been employed as conductivecoatings for nonconductive materials. For example, U.S. Pat. No.4,035,265, which issued to Saunders on July 12, 1977, disclosesconductive paint compositions containing both graphite and carbon blackalong with air-hardenable binder. These paints are suitable forapplication to the walls of a building for use as a heating element.

U.S. Pat. No. 4,090,984, which issued to Lin et al. on May 23, 1978,teaches a semi-conductive coating for glass fibers comprising (a) apolyacrylate emulsion; (b) electrically conductive carbon blackdispersion and (c) a thixotropic gelling agent. The conductive carbonblack dispersions employed are those comprising electrically conductivecarbon black dispersed in from about 3 to about 4% by weight of asuitable dispersing agent.

U.S. Pat. No. 4,239,794, which issued to Allard on Dec. 16, 1980,teaches dispersing a conductive carbon black in a latex binder with aselected dispersing agent, then impregnating this carbon blackdispersion into a nonwoven fibrous web followed by drying any residualwater, leaving a thin coating of carbon black dispersed on the surfacesof said fibers.

U.S. Pat. Nos. 4,619,741; 4,684,560 and 4,724,005, which issued to KarlL. Minten and Galina Pismennaya, on Oct. 28, 1986; Aug. 4, 1987; andFeb. 9, 1988, respectively, teach a process of electroplating thethrough holes of a PWB which is a significant improvement over the knownelectroless techniques. By this process, a liquid dispersion of carbonblack particles is first applied to the nonconductive portions of thethrough holes; then the liquid dispersion medium is separated (i.e.,evaporated) from the carbon black particles, thereby depositing asubstantially continuous layer of carbon black particles on thenonconductive surfaces of the through holes; and next a substantiallycontinuous metal layer is electroplated over the deposited carbon blacklayer. This process of Minten and Pismennaya has several advantages overthe known electroless techniques including the elimination of thepreactivator, the Pd/Sn activator and the accelerator; less possibilityof pollution problems; better bath stability; and fewer possible sidereactions. This disclosure of the above-mentioned U.S. Patents of Mintenand Pismennaya is incorporated herein by reference in their entirety.

Improvements and modifications of this Minten and Pismennaya process areshown in U.S. Pat. No. 4,622,107 (Piano); U.S. Pat. No. 4,622,108(Polakovic and Piano); U.S. Pat. No. 4,631,117 (Minten, Battisti, andPismennaya); and U.S. Pat. No. 4,718,993 (Cupta and Piano); U.S. Pat.No. 4,874,477 (Pendleton) and U.S. Pat. No. 4,897,164 (Piano andGalvez). The first of these patents teaches the use of a gas-formingcompound (e.g. sodium carbonate) to remove loose or easily removablecarbon black particles in the through holes. The second of these patentsteaches the contacting of an alkaline hydroxide preconditioning solutionto the through hole walls before application of the carbon blackdispersion so that the carbon black dispersion will have better adhesionto the walls. The third listed patent teaches the use of this carbonblack dispersion as a preactivator for electroless plating of thethrough holes. The fourth teaches the use of an alkaline silicatesolution before the carbon black dispersion. The fifth patent teachesthe use of an aqueous polyelectrolyte homopolymer conditioner solutionbefore the carbon black dispersion bath. The sixth patent teaches theuse of an alkaline borate solution to remove excess carbon blackmaterial on the rims and inner metal walls of the PWB through hole wallswhich might cause an undesirable, uneven plated surface to result. Thesesix U.S. Patents are incorporated herein by reference in theirentireties.

One problem present with multilayer PWB through holes is that thedrilling of the holes causes resin smear on the exposed conductive metal(e.g., copper) inner layers on the holes. The resin smear may act as aninsulator between the later plated-on metal in the through hole andthese inner metal layers. Thus, this smear may result in poor electricalconnections. The smear should be removed (i.e., "desmeared") before theplating-on operation.

Various alkaline permanganate treatments have been used as standardmethods for desmearing surfaces of printed wiring boards including thethrough holes of printed wiring boards. Such permanganate treatmentshave been employed for reliably removing smear/drilling debris andtexturizing or micro-roughening exposed PWB epoxy surfaces. This lattereffect significantly improves copper-to-epoxy resin adhesion.

Generally, permanganate treatment involves three different treatmentsolutions used sequentially. They are (1) a solvent swell solution, (2)a permanganate desmear solution, and (3) a neutralization solution.Typically, a printed wiring board is dipped or otherwise exposed to eachsolution with deionized water rinse baths employed between each of thesethree treatment solutions.

Numerous U.S. and foreign patents and published foreign patentapplications have issued which teach different permanganate desmearingand neutralization compositions and/or desmearing or neutralizationoperations. U.S. Pat. No. 3,962,496 (Leech) teaches of a hydrazineneutralizer solution containing a sequestering agent (e.g.ethylenediamine tetraacetic acid, sodium tartrate, and triethanolamine)and a pH adjustor (e.g. sodium hydroxide, potassium hydroxide, andsodium carbonate).

U.S. Pat. No. 4,042,729 (Polichette et al.) and U.S. Pat. No. 4,073,740(Polichette et al.) teach a composition for treating a resinous surfaceto later receive a deposit of electrolessly-formed metal, saidcomposition comprising water, permanganate ion and manganate ion,wherein the molar ratio of manganate ion to permanganate ion is up to1.2 to 1 and said composition having a pH in the range of 11 to 13.

U.S. Pat. No. 4,054,693 (Leech et al.) teaches a process of treating aresinous surface by first contacting that surface with the samepermanganate ion/manganate ion solution as used in the preceding twopatents, then neutralizing the treated resin surface with an aqueoussolution comprising hydrazine and then following that neutralizationwith metallizing that resinous surface with an electroless metaldeposition bath.

U.S. Pat. No. 4,233,344 (Braach) teaches treating a composite substratewith a copper-type colloidial system to cause activation of thenonconductive portions thereof for electroless metal deposition, andthereafter treating the activated substrate with an adhesion promoter(i.e., hydrazine hydrate, ammonium persulfate, or alkali hydroxide)prior to electroless metal deposition.

U.S. Pat. No. 4,425,380 (Nuzzi et al.) teaches a process for preparing aresinous substrate for subsequent metallization, said process comprisingfirst contacting the substrate with an alkaline permanganate treatingsolution, then contacting said substrate with a water-soluble compound(e.g., tin chloride, sodium bisulfite, hydrochloric acid, orhydroxylamine hydrochloride) to reduce any manganese residues depositedon said substrate to a low oxidation state, and finally contacting saidsubstrate with an alkaline hydroxide solution to remove essentially allof said manganese residues.

U.S. Pat. No. 4,430,154 (Stahl et al.) teaches a process for makingprinted circuit boards involving the steps of removing an adhesivecoating by treating the board with an aqueous solution containingpotassium permanganate and sodium hydroxide, and thereafter treated withan aqueous solution of hydrochloric acid or hydrazine hydrate.

U.S. Pat. No. 4,515,829 (Deckert et al.) teaches an overall process formanufacturing a printed circuit board having a plurality of metal platedholes interconnecting at least two circuits, including the steps ofdrilling holes in an epoxy board, forming the circuits, contacting thehole walls with an aqueous alkaline oxygenated epoxy solvent at a pHgreater than 10, then contacting the holes with an aqueous alkalinepermanganate solution at an elevated temperature and a pH in excess of13, and also contacting the hole walls with a reducing agent solution.

U.S. Pat. No. 4,592,852 (Courduvelis et al.) teaches an alkalinecomposition to improve the adhesion of plastics to electroless metaldeposits, said composition containing a source of permanganate ions anda secondary oxidant selected from the group consisting of chlorine,bromine, ozone, hypochlorite salts, metaperiodate salts andtrichloro-s-triazinetrione salts.

U.S. Pat. No. 4,592,929 (Tubergen et al.) teaches a process for themetallization of a plastic which includes the steps of first treatingthe plastic with a liquid permanganate oxidant solution, then contactingthe plastic with a solution containing a reducing agent, a pH adjustorto render the reducing agent active, and a surface active agent insufficient concentration to reduce the surface tension of the solutionto 50 dynes per centimeter or less.

U.S. Pat. No. 4,601,784 (Krulik) teaches an aqueous alkaline sodiumpermanganate solution comprising water, an alkali metal hydroxide,sodium permanganate, and 0.1 to about 3.0 moles of K⁺, Cs⁺, Rb⁺ ions, ormixtures thereof, per mole of permanganate ion.

U.S. Pat. No. 4,629,636 (Courduvelis et al.) teaches a process toimprove the adhesion of a plastic to an electroless metal depositwherein said plastic is contacted with an alkaline permanganate solutionwhich contains permanganate ions, manganate ions, and a secondaryoxidant; the secondary oxidant being added at controlled intervals tokeep the ratio of permanganate ion concentration to the sum ofpermanganate and manganate ion concentrations above about 0.5.

U.S. Pat. No. 4,698,124 (Krulik) teaches a method for regenerating spentpermanganate ions in a permanganante-containing etchant compositioncomprising periodically adding an oxidizer selected from the groupconsisting of an inorganic peroxy disulfate, mixtures of an inorganicperoxy disulfate, and an inorganic hypochlorite, and mixtures of anorganic peroxy disulfate, and an inorganic chlorate in an amount tooxidize essentially all of the nonpermanganate manganese species in thecomposition to permanganate.

Japanese Patent No. 81-003373 (which issued on Jan. 24, 1981) andJapanese Patent No. 81-015736 (which issued on Apr. 11, 1981) teach theuse of alkaline solutions of potassium permanganate and sodium orpotassium hypochlorite in the treatment of ABS resins prior toelectroless metal plating. The alkaline solutions include those having apH in the range of 11.0 to 12.35 and 12.0 to 13.5, respectively.

Japanese Kokai No. 79-055933 and Japanese Kokai 79-117,328, the latterpublished on Sept. 12, 1979, teach an electroless plating on plasticsprocess involving etching the plastics with aqueous solution containingpotassium permanganate and persulfate prior to electroless metalplating. All of the above U.S. and foreign patents and patentpublications are incorporated herein by reference in their entireties.

It is a primary object of this invention to provide an improvedelectroplating process for applying a conductive metal layer to anonconducting material such as the through hole walls of printed wiringboards over the process disclosed in the above-noted Minten andPismennaya patents.

It is also an object of the present invention to provide a unifiedpermanganate desmearing operation with the carbon black dispersionpreplating operation disclosed in the above-noted Minten and Pismennayapatents.

It is another object of the present invention to provide a unifiedpermanganate desmearing/carbon black dispersion preplating operationwhereby the neutralization treatment of the desmearing operation and theconditioning treatment of the preplating operation is combined into onestep.

It is still another object of this invention to provide an even moreeconomical and environmentally safe process for applying a conductivemetal layer to the surfaces of nonconducting layers of printed wiringboards than presently known combined permanganate/electroless processes.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention accomplishes the foregoing objects byproviding a process for electroplating a conductive metal layer to thesurface of a nonconductive material which comprises the steps:

(a) contacting said substrate surface with an alkaline permanganatesolution for an effective time and at an effective concentration and atan elevated temperature to prepare the substrate surface for a metallayer to be later electroplated thereto;

(b) then contacting said substrate surface with an aqueousneutralizer/conditioner solution, said solution comprising water, atleast one neutral or acidic reducing agent, and at least onepolyelectrolyte polymer conditioner;

(c) then contacting said substrate surface with a liquid dispersion ofcarbon black comprised of:

(1) carbon black particles having an average particle size of less thanabout 3.0 microns in said dispersion;

(2) an effective dispersing amount of a surfactant which is compatiblewith said carbon black; and

(3) a liquid dispersing medium, wherein the amount of carbon black issufficient to coat substantially all of the nonconducting substratesurface and is less than about 4% by weight of said liquid dispersion;

(d) separating substantially all of the liquid dispersing medium fromsaid applied dispersion, thereby depositing said carbon black particlesin a substantially continuous layer on said nonconducting substratesurface; and

(e) electroplating a substantially continuous metal layer over thedeposited carbon black layer on said nonconducting substrate surface.

The process of this invention is particularly useful for applying aconductive metal surface such as copper to the nonconducting portions ofthrough hole walls of printed wiring boards. These printed wiring boardsare usually composed of a nonconductive layer (e.g., epoxy resin/glassfiber mixture) positioned between two conductive metal layers (e.g.,copper or nickel plates or foils) or a multiplicity of said alternatinglayers. Applying a conducting metal layer over said nonconductingportions of said through hole walls electrically connects the conductivemetal layers. However, the process of this invention is effective forelectroplating a conductive metal onto the surface of a nonconductingmaterial (e.g., nonconductive plastics or resins or ceramics) ofvirtually any shape or surface area.

The present invention is also directed to said aqueousneutralizer/conditioner solution, as a composition-of-matter, comprisingwater, at least one neutral or acidic reducing agent, and at least onepolyelectrolyte polymer conditioner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As stated above, one preferred embodiment of the present invention ispreparing the through hole walls of a printed wiring board for theapplication of an electroplated layer of metal. Printed wiring boards(also known as printed circuit boards) are generally laminated materialscomprised of two or more plates or foils of copper, which are separatedfrom each other by a layer of nonconducting material. Although copper isgenerally used as the electroplating metal in printed wiring boards,those skilled in the art will recognize that other metals such asnickel, gold, palladium, silver and the like can also be electroplatedby the process of this invention. The nonconducting layer or layers arepreferably an organic material such as an epoxy resin impregnated withglass fiber particles. However, the nonconducting layer may also becomprised of thermosetting resins, thermoplastic resins, and mixturesthereof, with or without reinforcing materials such as fiberglass andfillers.

Suitable thermoplastic resins include the acetal resins; acrylics, suchas methyl acrylate; cellulosic resins, such as ethyl cellulose,cellulose acetate, cellulose propionate, cellulose acetate butyrate,cellulose nitrate, and the like; chlorinated polyethers; nylon;polyethylene; polypropylene; polystyrene; styrene blends, such asacrylonitrile styrene copolymers and acrylonitrile-butadiene-styrene(ABS) copolymers; polycarbonates; polychlorotrifluoroethylene; and vinylpolymers and copolymers, such as vinyl acetate, vinyl alcohol, vinylbutyral, vinyl chloride, vinyl chlorideacetate copolymer, vinylidenechloride and vinyl formal; and the like.

Suitable thermosetting resins include alkyl phthalate; furane;melamine-formaldehyde; phenol formaldehyde and phenol-furfuralcopolymers; alone or compounded with butadiene acrylonitrile copolymeror acrylonitrile-butadiene-styrene (ABS) copolymers; polyacrylic esters;silicones; urea formaldehydes; epoxy resins; polyimides; alkyl resins;glyceryl phthalates; polyesters; and the like.

In many printed wiring board designs, the electrical pathway or patternrequires a connection between the separated copper plates at certainpoints in the pattern. This is usually accomplished by drilling holes atthe desired locations through the laminate of copper plates and thenonconducting layer and then connecting the separate metal plates. Thehole diameters of printed wiring boards generally range from betweenabout 0.5 and about 10 millimeters in diameter, and preferably fromabout 1 to about 5 millimeters.

After drilling these through holes, it may be desirable to deburr theholes to make the hole walls relatively smooth. In the case ofmultilayer printed wiring boards, it may also be desirable to subjectthe boards to desmear or etchback operations to clean the inner copperinterfacing surfaces of the through holes.

According to the present invention, printed wiring boards having drilledthrough holes are treated with a specific alkaline permanganatetreatment before contacting the PWB with a carbon black dispersionpreplating treatment. This specific alkaline permanganate treatmentgenerally comprises three separate treatments--an optional epoxy resinswell treatment, an alkaline permanganate treatment, and aneutralizer/conditioner treatment.

The optional epoxy resin swell treatment involves contacting the PWBwith a suitable swelling agent. This may include an aqueous alkalinesolution containing an alkali metal hydroxide and at least one glycolether or other suitable solvent. Alternatively, the swelling agent maybe a nonaqueous solvent (e.g., N-methylpyrrolidone). Generally, it ispreferred to use a combination of an alkali metal hydroxide and a glycolether mixture. One preferred mixture of glycol ethers is a mixture ofbutyl cellosolve, butyl carbitol, and ethylene glycol. Butyl cellosolveis also known as 2-butoxy ethanol or ethylene glycol monobutyl ether andhas the chemical formula: C₄ H₉ OC₂ H₄ OH. Butyl carbitol is also knownas diethylene glycol monobutyl ether; butoxydiethylene glycol; or2-(2-butoxyethoxy)ethanol and has the chemical formula: C₄ H₉ O(C₂ H₄O)₂ H. Ethylene glycol is also known as glycol or 1,2-ethanediol and hasthe chemical formula: HOC₂ H₄ OH.

Preferably, this epoxy resin swell treatment is carried out by dippingthe PWB having the drilled through holes into an aqueous bath containingthe above-noted ingredients at an elevated temperature (e.g., about40°-80° C.) for a short period of time (e.g., about 3 to 15 minutes). Apreferred epoxy swell product is available as PERMOLIN 701 by Olin HuntSpecialty Products Inc. of West Paterson, N.J.

Next, the printed wiring board is contacted with an alkalinepermanganate solution, preferably containing an alkaline hydroxide, analkaline permanganate, an alkaline manganate (produced in situ), andoptionally, a secondary oxidizer such as alkaline hypochlorite oralkaline persulfate. Other ingredients may also be used. The presentinvention encompasses the use of any alkaline permanganate treatmentconventionally used for this purpose which will be compatible withneutralizer/conditioner solution employed in the next step. The term"alkaline", as used herein in both the specification and the claims,means compounds containing ammonium, alkali metal, and alkaline earthmetal ions.

One preferred alkaline permanganate treatment is to place the drilledPWB in an aqueous bath containing an aqueous alkaline permanganatesolution for about 5 to 30 minutes, heated to a temperature of about 50°C. to 90° C. at a concentration to effectively desmear the through holesof the PWB and this prepared the through holes for the application ofthe electroplated-on metal.

The alkaline permanganate treatment bath may be made from the followingingredients in the following amounts:

    ______________________________________                                                                       Most                                                       General Preferable Preferable                                                 Amounts Amounts    Amounts                                                    (% by Wt.)                                                                            (% by Wt.) (% by Wt.)                                     ______________________________________                                        Sodium or Potassium                                                                         2.5-6     3-5        4                                          Hydroxide                                                                     Sodium or Potassium                                                                         5-7       5.5-6.5    6                                          Permanganate                                                                  Sodium or Potassium                                                                         0.1-1.0   0.2-0.4    0.3                                        Hypochlorite                                                                  Water         Balance   Balance    Balance                                    ______________________________________                                    

A preferred alkaline permanganate product is available as PERMOLIN 702by Olin Hunt Specialty Products Inc. of West Paterson, N.J.

Instead of employing a secondary oxidizer in this treatment step, it maybe desirable to use an electrolytic regeneration of permanganate.

Next, the permanganate-treated PWB is contacted with an aqueousneutralizer/conditioner solution comprised of water, at least oneneutral or acidic reducing agent, optionally, at least one pH adjustor,and at least one polyelectrolyte polymer conditioner. This solutionneutralizes any remaining permanganate residues remaining on the PWBhole surfaces and ensures that substantially all of the hole wallglass/epoxy surfaces are properly prepared to accept a continuous layerof the subsequently applied carbon black particles.

One preferred method of contacting the permanganate-treated PWB withthis neutralizer/conditioner solution is to place the treated PWB in anaqueous bath containing the neutralizer/conditioner ingredients forabout 1 to 20 minutes at a temperature from about 40° C. to 80° C.

Preferred acidic reducing agents include dihydrazine sulfate andhydroxylamine sulfate and other salts thereof. It may also be preferredto use an additional acidifying means or pH adjustor such ashydrochloric acid. A preferred permanganate neutralizer/conditionertreatment bath may contain the following ingredients in the followingamounts:

    ______________________________________                                                                       Most                                                       General Preferable Preferable                                                 Amounts Amounts    Amounts                                                    (% by Wt.)                                                                            (% by Wt.) (% by Wt.)                                     ______________________________________                                        Dihydrazine Sulfate                                                                          1.0-2.5  1.2-2.0    1.5                                        Hydrochloric Acid                                                                           0.2-2     0.8-1.2    1.0                                        Polyelectrolyte                                                                             0.01-0.2  0.06-0.15  0.1                                        Polymer Conditioner                                                           Water         Balance   Balance    Balance                                    ______________________________________                                    

Preferred polyelectrolyte polymer conditioners include cationicpolyelectrolyte polyacrylamide homopolymer resin such as MAGNIFLOCcationic resins available from American Cyanimid Company of Wayne, N.J.and cationic polyamine homopolymer resins such as ETADURIN 21 availablefrom Akzo Chemical Company of Chicago, Ill.

It is also preferred to rinse the above-treated boards in at least onewater rinse between the epoxy swell treatment and the alkalinepermanganate treatment and between the alkaline permanganate treatmentand the neutralizer/conditioner treatment. It is also preferred toemploy at least one water rinse bath after the neutralizer/conditionertreatment.

The liquid carbon black dispersion is next applied to or contacted withthe neutralized/conditioned PWB. This dispersion contains three criticalingredients, namely, carbon black, one or more surfactants capable ofdispersing the carbon black and a liquid dispersing medium such aswater. The preferred method of applying the dispersion to the PWBinclude immersion, spraying or other methods of applying chemicals usedin the PWB industry. A single working bath is sufficient for applyingthis carbon black dispersion; however, more than one bath may be usedfor rework or other purposes.

In preparing this liquid dispersion, the three critical ingredients andany other preferred ingredients are thoroughly mixed together to form astable dispersion. This may be accomplished by subjecting a concentratedform of the dispersion to ball milling, colloidal milling, high-shearmilling, ultrasonic techniques or other standard blending techniques.The thoroughly mixed dispersion is later diluted with more water whileagitating to the desired concentration for the working bath. Thepreferred method of mixing is ball milling a concentrated form of thedispersion in a container having glass, mineral, or plastic beadstherein for a period of about 1 to about 24 hours. This thorough mixingallows for the carbon black particles to be intimately coated or wettedwith the surfactant. This mixed concentrate is then mixed with water orsome other liquid dispersing medium to the desired concentration. Theworking bath is preferably kept agitated during both the diluting andapplying steps to aid in keeping the dispersion stable.

As stated above, the carbon black particles should have an averageparticle diameter below about 3 microns while in the dispersion. It isdesirable to have this average particle diameter of carbon black assmall as possible to obtain the desired plating characteristics ofsubstantially even plating and no plating pullaways. It is preferredthat the carbon black particles have an average particle diameter fromabout 0.1 to about 3.0 microns, more preferably from 0.2 to about 2.0microns, when in said dispersion. The term "average particle diameter"as employed herein in both the specification and claims refers toaverage mean diameter of the particles (the average by number). Theaverage mean diameter in the dispersion may be determined through theuse of either a NiComp Model 270 submicron particle size analyzer(Version 3.0) or a HIAC PA-720 automatic particle size analyzer (bothavailable from the HIAC/ROYCO Instrument Division of Pacific Scientificof Menlo Park, Calif.).

All types of carbon blacks may be used for this invention including thecommonly available furnace blacks. However, it is preferred to utilizecarbon blacks which are initially acidic or neutral, i.e., those whichhave a pH of between about 1 and about 7.5 and more preferably betweenabout 2 and about 4 when slurried with water. Carbon black particles ofthe preferred type contain between about 1% and about 10% by weight ofvolatiles and have an amorphous structure.

These preferred carbon black particles are also very porous andgenerally their surface areas are from between about 45 to about 1100,and preferably between about 300 to about 600, square meters per gram asmeasured by the BET method (method of Brunauer-Emmett-Teller).

Illustrative carbon blacks suitable for use of this invention includeCabot XC-72R Conductive, Cabot Monarch 800, Cabot Monarch 1300, allmanufactured by Cabot Corporation of Boston, Mass. Other suitable carbonblacks include Columbian T-10189, Columbian Conductex 975 Conductive,Columbian CC-40-220, and Columbian Raven 3500, all available fromColumbian Carbon Company of New York, N.Y. Monarch 800 and Raven 3500are the two most preferred carbon blacks because of their ease ofdispersion and low pH.

The term "liquid dispersing medium" as used herein in the presentspecification and claims includes water and polar organic solvents (bothprotic and aprotic). Suitable protic polar organic solvents may includelower alcohols (C₁ -C₄) such as methanol, ethanol, isopropanol andisobutanol; polyhydric alcohols such as glycols (i.e. triethyleneglycols); ether-alcohols such as cellosolve; organic acids, such asformic acid and acetic acid; acid derivatives such as trichloroaceticacid; and sulfonic acids such as methane sulfonic acid. Suitable aproticpolar organic solvents include aldehydes such as acetaldehyde; ketonessuch as acetone; aprotic aromatic solvents such as toluene and mineralspirits; aprotic halogenated hydrocarbons such as dichlorofluoromethaneand dichlorodifluoromethane (FREON); dimethylformamide (DMF);N-methylpyrrolidone; dimethylsulfoxide (DMSO); and esters of carboxylicacids such as methylformate, ethylacetate and cellosolve acetate. Thepreferred liquid dispersing medium is water because of cost and ease ofuse considerations. It is preferred to utilize deionized water which isfree of lime, fluorine, iodine and other impurities normally found intap water, in order to minimize interference of foreign ions during thesubsequent electroplating step.

In addition to the water and carbon black, a third critical ingredientis needed in the dispersion, namely, a surfactant capable of dispersingsaid carbon black in said liquid dispersing medium (i.e., compatiblewith said carbon black and liquid dispersing medium). One or more ofthese is added to the dispersion in order to enhance the wetting abilityand stability of the carbon black and permit maximum penetration by thecarbon black within the pores and fibers of the nonconducting layer.Suitable wetting agents include anionic, nonionic and cationicsurfactants (or combinations thereof such as amphoteric surfactants).The surfactants should be soluble, stable and preferably nonfoaming inthe liquid carbon black dispersion. In general, for a polar continuousphase as in water, the surfactants should preferably have a high HLBnumber (8-18). The preferred type of surfactant will depend mainly onthe pH of the dispersion. If the total dispersion is alkaline (i.e., hasan overall pH in the basic range), it is preferred to employ an anionicor nonionic surfactant. Acceptable anionic surfactants include sodium orpotassium salts of naphthalene sulfonic acid such as DARVAN No. 1 (R.T.Vanderbilt Co.), ECCOWET LF (Eastern Color and Chemical), PETRO AA,PETRO ULF (Petro Chemical Co., Inc.), and AEROSOL OT (AmericanCyanamid). Preferred anionic surfactants include neutralized phosphateester-type surfactants such as MAPHOS 55, 56, 8135, 60A, L6 (MazerChemicals Inc.). The most preferable anionic surfactant for a liquidcarbon black dispersion is MAPHOS 56. Suitable nonionic surfactantsinclude ethoxylated nonyl phenols such as POLY-TERGENT® B-Series (OlinCorporation) or alkoxylated linear alcohols such as POLY-TERGENTSL-Series (Olin Corporation).

If the total dispersion is acidic, it is preferred to employ selectedanionic surfactants or cationic surfactants. An acceptable group ofanionic surfactants would be the sodium or potassium salts ofnaphthalene sulfonic acid described above. Acceptable cationicsurfactants include cetyl dimethyl benzyl ammonium chloride such asAMMONYX T (Onyx Chemical Corporation); an ethanolated alkylguanidineamine complex such as AEROSOL C-61 (American Cyanamid); lipocals;dodecyldiphenyl oxide disulfonic acid (DDODA) such as DOWFAX 2Al (DowChemical); a sodium salt of DDODA such as STRODEX (Dexter ChemicalCorporation); and salts of complex organic phosphate esters. Preferredsurfactants include amphoteric potassium salts of a complex amino acidbased on fatty amines such as MAFO 13 and cationic ethoxylated soyaamines such as MAZEEN S-5 or MAZTREAT (Mazer Chemicals Inc.).Combinations of surfactants may be employed. The term "surfactant", asused herein for making the carbon black dispersion, may include otherforms of dispersing agents or aids such as low molecular weightpolyelectrolytes and polymers.

The amount of carbon black in the dispersion should be less than about4% by weight of the dispersion, preferably, less than about 2% byweight. It has been found that the use of higher concentrations ofcarbon blacks provide undesirable plating characteristics. In the sameregard, the solids content (i.e. all of the ingredients other than theliquid dispersing medium) is preferably less than 10% by weight of thedispersion, more preferably, less than about 5.6% by weight.

The three above-noted critical components of the present invention,namely, the carbon black, liquid dispersing medium and surfactant, maybe employed alone to form a liquid dispersion. In some situations, itmay be desirable to add other preferred ingredients to this dispersion.

One additional preferred component of the liquid carbon black-containingdispersion is a strong basic material such as an alkaline hydroxide.Suitable strong basic materials include alkali metal hydroxides such aspotassium hydroxide, sodium hydroxide, and lithium hydroxide. Ammoniumhydroxide or alkaline earth metal hydroxides such as calcium hydroxidemay also be employed, if desired. Potassium hydroxide is the mostpreferred strong basic material. The term "alkaline hydroxide" is usedthroughout the description and claims to identify these strong basicmaterials. Sufficient alkaline hydroxide may be added to the liquidcarbon black dispersion in a proportion sufficient to increase the pH ofthe resulting carbon black-containing dispersion to between about 10 andabout 14, and preferably between about 10 and about 12.

Following is a typical formulation of a suitable aqueous alkalinedispersion of carbon black showing the general range of proportions aswell as the preferred range of proportions for the various components:

    ______________________________________                                        Component     General Range                                                                              Preferred Range                                    ______________________________________                                        Carbon Black  0.1-4% by wt.                                                                              0.15-2% by wt.                                     Surfactant    0.01-4%      0.05-2%                                            Alkaline Hydroxide                                                                          0-1%         0.4-0.8%                                           Water         Balance      Balance                                            ______________________________________                                    

The liquid dispersion of carbon black is typically placed in a suitablyagitated vessel and the printed wiring board to be treated is immersedin, sprayed with or otherwise contacted with the liquid dispersion. Thetemperature of the liquid dispersion in an immersion bath is maintainedin the range of between about 15° C. and about 35° C. and preferablybetween about 20° C. and about 30° C., while the conditioned printedwiring board is immersed therein. The period of immersion generallyranges from about 1 to 10, and preferably from about 3 to 5 minutes.During immersion, the liquid carbon black-containing dispersionpenetrates the holes of the printed wiring board and wets and contactsthe glass fiber as well as the epoxy resin which forms the components ofthe insulating layer. The immersed board is then removed from the liquidcarbon black-containing dispersion bath.

The carbon black-covered board is then subjected to a step wheresubstantially all (i.e., more than about 95% by weight) of the water inthe applied dispersion is removed and a dried deposit containing carbonblack is left in the holes and on other exposed surfaces of thenonconducting layer. This may be accomplished by several methods such asby evaporation at room temperature, by a vacuum, or by heating the boardfor a short time at an elevated temperature, or by other equivalentmeans. Heating at an elevated temperature is the preferred method.Heating is generally carried out for between about 30 seconds and 45minutes at a temperature of from about 75° C. to 120° C., morepreferably from about 80° C. to 98° C. To insure complete coverage ofthe hole walls, the procedure of immersing the board in the liquidcarbon black dispersion and then drying may be repeated one or moretimes.

This drying step yields a board which may be completely coated with thecarbon black dispersion. This dispersion is not only coated on thedrilled hole surfaces, which is desirable, but also entirely coats thecopper plate or foil surfaces which is undesirable. Thus prior to manysubsequent operations all carbon black must be removed from the copperplate or foil surfaces.

As an optional feature of the present invention, the dried deposit ofcarbon black in the through holes is then contacted with an aqueousalkaline solution containing an alkali metal borate. The preferredalkali metal borate is sodium borate. The preferred pH range of thisalkaline solution is from about 9.5 to 11.0. The preferred bathtemperature is from about 20° C. to 50° C. The functions of this stepinclude removing excess carbon black material on the rims and innermetal walls of the PWB through holes and remove any loose carbon blackparticles from the through hole walls which might cause an undesirableuneven plated surface to result. The alkali metal borate also increasesthe surface porosity of the carbon black.

If used, the amount of alkali borate should be sufficient to removesubstantially all of the loose or easily removable carbon blackparticles from the areas of the through holes. The preferredconcentration may vary from about 2 to 50 grams per liter of wateremployed. This contacting step may be carried out by placing the PWB inan aqueous bath containing the alkali metal borate at a temperature fromabout 20° C. to 50° C. for about 20 seconds to 5 minutes.

The further removal of the carbon black, specifically from the outercopper surfaces including, especially, the rims of the drilled holeswhile leaving the coating intact on the glass fibers and epoxy surfaceof the hole walls, may be achieved by the employment of a microetchbath. Generally, this treatment is carried out at a temperature of about20° C. to 30° C. for 35 seconds to about 3 minutes. One suitable sodiumpersulfate-based microetch is "BLACKHOLE MICROCLEAN I" available fromOlin Hunt Specialty Products Inc. This product is preferably combinedwith sufficient sulfuric acid to make a microetch bath containing100-300 grams of sodium persulfate per liter of deionized water andabout 1 to 10% by weight sulfuric acid. The mechanism by which thismicroetch works is by not attacking the carbon black material depositedon the copper foil directly, but rather to attack exclusively the firstfew atomic layers of copper directly below which provides the adhesionfor the coating. Hence, the fully coated board is immersed in themicroetch solution to "flake" off the carbon black from the coppersurfaces in the form of micro-flakelets. These micro-flakelets areremoved from the microetch bath either by filtration through a pump orvia a weir type filter arrangement commonly used in the PWB industry.The liquid carbon black dispersion, the alkali metal borate treatment,the microetch treatment, and the intermittent water rinses arepreferably carried out by immersing the PWB in baths constructed ofpolypropylene or polyvinyl chloride (PVC) and kept agitated by arecirculation pump or pumped in air.

After the microetch step and a subsequent water rinse, the PWB may noweither proceed to the photoimaging process and later be electroplated orbe directly panel electroplated. It may be preferred to further cleanthe PWB with a citirc acid anti-tarnish solution or any other acidcleaner solution or both after the above microetch step.

The thus treated printed wiring board is then ready for electroplatingoperation which includes immersing the PWB in a suitable electroplatingbath for applying a copper coating on the hole walls of thenonconducting layer.

The present invention contemplates the use of any and all electroplatingoperations conventionally employed in applying a metal layer to thethrough hole walls of a PWB. Therefore, this claimed invention shouldnot be limited to any particular electroplating bath parameters.

A typical copper electroplating bath is comprised of the followingcomponents in the following proportions:

    ______________________________________                                                      General      Preferred                                          Component     Proportions  Proportions                                        ______________________________________                                        Copper (as metal)                                                                           2-3     oz/gal   2.25-2.75                                                                            oz/gal                                  CoPPer Sulfate                                                                              5-10    oz/gal   6-9    oz/gal                                  98% Concentrated                                                                            23-32   oz/gal   27-30  oz/gal                                  H.sub.2 SO.sub.4 (by weight)                                                  Chloride Ion  20-100  mg/l     40-60  mg/l                                    ______________________________________                                    

The electroplating bath is normally agitated and preferably maintainedat a temperature of between about 20° C. and 25° C. The electroplatingbath is provided with anodes, generally constructed of copper, and theprinted wiring board to be plated is connected as a cathode to theelectroplating circuit. For example, a current of about 30 amps persquare foot is impressed across the electroplating circuit for a periodof between about 40 and 60 minutes in order to effect copper plating onthe hole walls of the nonconducting layer positioned between the twoplates of copper up to a thickness of about 1 mil ±0.2 mil. This copperplating of the hole wall provides a current path between the copperlayers of the printed wiring board. Other suitable electroplatingconditions may be employed, if desired. Other electroplating bathcompositions containing other copper salts or other metal salts such assalts of nickel, gold, palladium, silver and the like may be employed,if desired.

The printed wiring board is removed from the copper electroplating bathand then washed and dried to provide a board which is further processed.For example, the PWB may be subjected to a tin-lead electroplatingoperation.

The following examples are presented to define the invention more fullywithout any intention of being limited thereby. All parts andpercentages are by weight unless otherwise explicitly specified.

PRINTED WIRING BOARD SPECIFICATIONS

Several similar laminated double-sided and multilayer printed wiringboards were treated in the following Examples and Comparisons. Thedouble-sided printed wiring boards were each comprised of two 35 micronthick copper plates secured by pressure to opposite sides of an epoxyresin-glass fiber layer. The epoxy resin-glass fiber layer for thesedouble-sided boards was about 1.55 mm thick for each board. Thesedouble-sided printed wiring boards were about 15 centimeters wide andabout 24 centimeters in length. There were about 100 to 200 holes, eachabout 1.0 millimeters in diameter, drilled through the copper plates andepoxy resin-glass fiber layer of each of these double-sided boards.

The multilayer printed wiring boards were comprised of four 35 micronthick copper plates secured by pressure fusing them with epoxyresin-glass fiber layers in an alternating fashion These printed wiringboards were about 15.24 centimeters wide and about 22.86 centimeters inlength. There were about 500 to about 1,000 holes, each about 1.0millimeter in diameter drilled through the copper plates and epoxyresin-glass fiber layer of each of these double-sided boards.

EXAMPLE 1

Drilled double-sided printed wiring boards described above were preparedfor copper electroplating their through holes by first mechanicalscrubbing the surfaces of the boards and then immersing them in thefollowing sequence of aqueous baths (each about 132 liters in volume,unless specified otherwise) for the indicated times:

1. Epoxy swell (5 minutes).

2. Rinse with tap water (1 minute).

3. Rinse with tap water (2 minutes).

4. Permanganate desmear (15 minutes).

5. Rinse with tap water (2 minutes).

6. Rinse with tap water (3 minutes).

7. Neutralizer-conditioner (5 minutes).

8. Rinse with tap water (1 minute).

9. Rinse with tap water (1 minute).

10. Carbon black preplating dispersion (4 minutes). (Dry at 93° C for 20minutes.)

11. Rinse with tap water (30 seconds).

12. Sodium persulfate microetch (1 minute).

13. Rinse with tap water (2 minutes).

14. Citric acid spray.

15. Acid Cleaner Solution (3 minutes)

16. Rinse with tap water (1 minute)

17. Rinse with tap water (1 minute)

18. Microetch (10 seconds)

19. Rinse with tap water (2 minutes)

20. 20% H₂ SO₄ (2 minutes)

21. Acid Copper Plating (5 minutes)

22. Rinse with tap water (2 minutes)

Bath 1 was an aqueous solution containing an epoxy resin swellformulation comprised of butyl cellosolve (6% by wt.), butyl carbitol(9% by wt.), ethylene glycol (7.5% by wt.), sodium hydroxide (about 2%by wt.), and the balance deionized water. The bath was heated to about60° C. to facilitate this swelling. This swell formulation is availableunder the trademark "PERMOLIN 701"by Olin Hunt Specialty Products Inc.of West Paterson, N.J.

Bath 4 was an aqueous solution containing potassium permanganate (6% bywt.), sodium hydroxide (4% by wt.), and sodium hypochlorite (0.3% bywt.) and the balance being deionized water which facilitate the removalof the epoxy resin smear over the inner layers of the drilled throughholes. The bath was heated to about 71° C. to facilitate smear removal.This smear removal formulation is available under the trademark"PERMOLIN 702"by Olin Hunt Specialty Products Inc. of West Paterson,N.J.

Bath 7 was an aqueous solution containing a permanganate neutralizerformulation comprised of dihydrazine sulfate (1.1% by wt.), hydrochloricacid (5% by wt.), a cationic homopolymer (0.1% by wt.), and balancebeing deionized water. The homopolymer was the polyaminepolyelectrolyte, ETADURIN 21. This solution both reduces the insolublemanganese salts, rendering them water soluble, and also prepares theglass surfaces in the through holes as a cationic entity for theadsorption of the anionic carbon black. Bath 7 was heated to about 60°C. to facilitate dissolution. ETADURIN 21 is available from AkzoChemical Company of Chicago, Ill.

Bath 10 is a room temperature deionized water bath containing the carbonblack preplating formulation. This bath consisted of: 0.06% by weightanionic surfactant [MAPHOS 56--an anionic surfactant produced by MazerChemicals of Gurnee, Ill. (90% by wt. surfactant, 10% by wt. H₂ O)],0.46% by wt. potassium hydroxide [solid potassium hydroxide pellets (86%by wt. KOH, 14% by wt. H₂ O)], 0.21% by wt. carbon black [RAVEN 3500carbon black produced by Cabot Corporation], and the balance of the bathwas deionized water. This carbon black dispersion was prepared bymilling a concentrated form of this dispersion in a pebble millcontaining stone pebbles so that the concentration of pebbles occupiedabout one third of the mill volume. The surfactant was dissolved indeionized water-potassium hydroxide to give a continuous phase. Then,the carbon black was added. Milling time was six hours. After milling,the concentrate was diluted with sufficient deionized water to make thedispersion in the above-indicated proportions.

After Bath 10, the boards were placed in a hot air recirculation ovenand heated to 93° C. for 20 minutes. This drying step removed the waterfrom the carbon coating on the boards, thereby leaving a dried depositof carbon black all over the boards and in the through holes of theboards. This drying promotes adhesion between the carbon black and thenonconductive surfaces of the boards.

Bath 12 was a room temperature aqueous bath and contained 200 grams ofsodium persulfate per liter of deionized water and 1.0% by wt. ofconcentrated sulfuric acid. Its function was to microetch the coppersurfaces of the boards so as to remove the deposited carbon black fromthe surfaces. It does not act on the epoxy-glass surfaces. This sodiumpersulfate microetch was made from "BLACKHOLE MICROCLEAN 1"and isavailable from Olin Hunt Specialty Products Inc. of West Paterson, N.J.

After treatment with this sequence of baths, the printed wiring boardswere sprayed with an anti-tarnish solution consisting of an aqueoussolution of citric acid. The printed wiring boards were passed throughthe spray on a conveyorized unit at 3 feet per minute where the solutionwas 43° C. A typical spray pressure was 14-15 psig.

Bath 15 was an aqueous acid cleaner solution containing sulfonic acid(about 10% by wt.) and a detergent (about 5% by wt.). This bath washeated to 45° C. to facilitate cleaning the copper surface. This coppercleaning solution is made by diluting VERSACLEAN 400 with deionizedwater (1:3 dilution). VERSACLEAN 400 is available from DuPont ElectronicChemicals of Wilmington, Del.

Bath 18 was a room temperature aqueous bath and contained 100 grams ofsodium persulfate per liter of deionized water and 1% by wt. ofconcentrated sulfuric acid. This bath and bath 20 are both part of astandard electroplating operation. Its function was to microetch thecopper surfaces of the boards to prepare a rough surface forelectroplating. It does not act on the epoxy-glass surfaces.

Bath 20 was an aqueous solution containing 20% by wt. concentratedsulfuric acid. Immersion of boards in this solution, prior to copperelectroplating, prevents excessive carry-over of water from the rinsebath to the copper electroplating bath, Bath 21.

Rinse baths 2, 3, 5, 6, 8, 9, 11, 13, 16, 17, 19, and 22 were employedto prevent the carryover of chemicals from one treatment bath into thenext. These rinse baths were at room temperature.

After the dilute sulfuric acid treatment in bath 20, the boards wereplaced in aqueous electroplating bath 21, provided with agitation meansand heating means, and which contained an electrolyte bath comprised ofthe following:

    ______________________________________                                        Aqueous Plating Bath Composition                                              Component              Proportion                                             ______________________________________                                        Copper (as metal)      2.5 oz./gal.                                           Copper Sulfate         6.2 oz./gal.                                           98% Concentrated H.sub.2 SO.sub.4                                                                    30 oz/gal.                                             (by weight)                                                                   Chloride Ion           40 mg/l                                                Water                  Balance                                                ______________________________________                                    

The printed wiring boards were connected as a cathode in theelectroplating vessel having a volume of about 284 liters. Copper barswere immersed in the electrolyte and connected to the cell circuits asanodes. The copper bars had a length of about 46 cm; a width of about 9cm, and a thickness of about 4 cm. Each face was about 414 square cm. Adirect current of 30 amps per square foot was impressed across theelectrodes in the electroplating bath for approximately 5 minutes. Thebath was maintained at a temperature of about 25° C. during this periodand agitation was effected by air sparging. At the end of this period,the printed wiring boards were disconnected from the electroplatingcircuit, removed from the electrolyte, washed with tap water, and dried.

Arbitrarily chosen areas, measuring 10 mm×18 mm, were cut from theprinted wiring boards, using a Di-Acro Houdaille No. 1 punch. Thethrough holes were then sectioned into semi-circles using an IsometII-1180 low speed saw. Back-lighting analysis was carried out byexamining the surface of the sectioned through holes with an opticalmicroscope. Each sample was illuminated from behind. The analysis ofabout 50 samples from these plated boards showed that about 87%±4% ofthe glass surfaces in the through hole samples had been electroplated.Our experience with back-lighting, as a technique to evaluate theprobability to completely electroplate the through hole surface, showsthat a minimum of 85% of the through hole must be copper covered afterfive minutes electroplating. Accordingly, the PWB's used in this examplewere successfully electroplated.

COMPARISON 1

The above process was repeated with similar double-sided PWB's exceptBath 7 did not contain the homopolymer polyelectrolyte ETADURIN 21, andBath 7 was followed by immersion of the PWB in a cleaner bath for fiveminutes (and subsequent cold water rinse for two minutes), thenimmersion in a conditioner bath for four minutes (and subsequent coldwater rinse for two minutes), followed by immersion in Bath 10 above.The cleaner bath, employed in this comparison, was an aqueous solutioncontaining a cleaner formulation principally comprised ofmonoethanolamine, ethylene glycol, and a nonionic surfactant in water toremove grease and other impurities from the hole wall surfaces of theboards. The cleaner bath was heated to about 60° C. to facilitate thiscleaning operation. This cleaner formulation is available as "BLACKHOLECLEANER 2" from Olin Hunt Specialty Products Inc. of West Paterson, N.J.

The conditioner bath was an aqueous solution of monoethanolamine andSandolec CF. This formulation prepares the boards and makes its throughholes more receptive to the carbon black dispersion. Sandolec CF is madeby Sandoz Chemical; this formulation is available as "BLACKHOLECONDITIONER" from Olin Hunt Specialty Products Inc. of West Paterson,N.J. Back-lighting analysis, as in Example 1, after 5 minuteselectroplating showed that about 93%±4% of the glass surfaces in throughhole samples had been electroplated.

However, this process required two additional treatment baths andsubsequent rinses. Thus, the overall cost of this process was greaterand the length of time needed to treat each PWB was longer. Accordingly,the process is a less attractive commercial process than that of Example1.

EXAMPLE 2

Example 1 was repeated using both double-sided and multilayer PWB'sexcept that sodium persulfate (0.3% by wt.) was substituted for thesodium hypochlorite in Bath 4, and the electrolytic copper depositiontime was extended from 5 to 55 minutes followed by 5 minuteselectrolytic tin-lead solder deposition. The following additional bathswere employed to carry out this electrolytic tin-lead solder deposition:

23. Rinse with tap water (2 minutes)

24. Acid pre-dip (2 minutes)

25. Tin-Lead plating (5 minutes)

26. Rinse with tap water (2 minutes)

Bath 24 was an aqueous solution containing 20% by wt. alkane sulfonicacid. Immersion of boards in this solution, prior to tin-leadelectroplating, prevents excessive carryover of water from the rinsebath to the tin-lead electroplating bath, Bath 25.

Rinse bath 23 was employed to prevent carryover of chemicals from onetreatment bath into the next. This rinse bath was at room temperature.

The tin-lead electroplating bath comprised of the following componentsin the following proportions:

    ______________________________________                                                     Range      Optimum                                               ______________________________________                                        Tin Metal      10-22 g/L    15-16 g/L                                         Lead Metal     5-11 g/L     7-8 g/L                                           Alkane Sulfonic Acid                                                                         16-30% by wt.                                                                              18-26% by wt.                                     Temperature    24-30° C.                                                                           27° C.                                     Current Density                                                                              10-40 A/sq. ft.                                                                            20 A/sq. ft.                                      ______________________________________                                    

At the end of this period, the printed wiring board was disconnectedfrom the electroplating circuit, removed from the electrolyte, washedwith tap water in room temperature rinse bath 26, and dried.

COMPARISON 2

The process outlined in Comparison 1 was repeated with both double-sidedand multilayered PWB's, except that sodium persulfate (0.3% by wt.) wassubstituted for sodium hypochlorite in Bath 4, and the board processingtime extended to include 55 minutes electrolytic copper deposition and 5minutes electrolytic tin-lead solder deposition.

COMPARISON 3

The process outlined in Comparison 2 was repeated with both double-sidedand multilayered PWB's, except that a permanganate neutralizer, PERMOLIN703 which is available from Olin Hunt Specialty Products Inc., wassubstituted for the conditioner-neutralizer in Bath 7 and theconditioner bath was removed. The cleaner bath was still employed.

An examination of the through holes of the resulting electroplatedprinted wiring boards of Example 2 and Comparisons 2 and 3 wereconducted, and the following defined parameters were evaluated:

    ______________________________________                                        Pullaway (PA):   Adhesion failure of the                                                       plated copper to the hole                                                     wall.                                                        Epoxy voids (EV):                                                                              Absence of plated copper on                                                   the resin surfaces.                                          Glass voids (GV):                                                                              Absence of plated copper on                                                   the glass surfaces.                                          Rim voids (RV):  Absence of plated copper at                                                   or just below the rim of                                                      the through holes.                                           Innerlayer       (for multilayer boards                                       contamination (ILC):                                                                           only) Residual carbon black                                                   material on the copper                                                        interconnects in the                                                          through hole.                                                Innerlayer fold  (for multilayer boards only)                                 void (ILFV):     Absence of plated copper at                                                   the resin innerlayer-copper                                                   interconnect.                                                ______________________________________                                    

A rating system for these defects was devised by assigning numbers from1 to 4 depending on the severity of the defect; 1 indicates no defectwas observed and 4 indicates the defect was observed on essentially allof the through holes inspected.

32 cross-sections with 5 through holes per cross-section totalling 160through holes were evaluated each for Example 2 and Comparisons 2 and 3.

The pullaway rating determined was after thermal shock testing in whichthe sample was floated in a molten solder bath at 288° C. for tenseconds.

    ______________________________________                                               PA    GV      EV      RV    ILC   ILFV                                 ______________________________________                                        Example 2                                                                     Double-sided                                                                           1.5     1       1     1     N.M.  N.M.                               Multilayer                                                                             1.2     1       1     1     1     1                                  Comparison 2                                                                  Double-sided                                                                           1.8     1       1     1     N.M.  N.M.                               Multilayer                                                                             1.2     1       1     1     1     1                                  Comparison 3                                                                  Double-sided                                                                           1.2     1.3     1     1     N.M.  N.M.                               Multilayer                                                                             1.2     2.1     1.1   1     1     1                                  ______________________________________                                         N.M. = Not measured                                                      

It is clearly evident by comparing the results of Example 2 toComparisons 2 and 3 that this invention produces results equal to orbetter than the processes of Comparisons 2 and 3 while offering aconsiderably shortened processing time and fewer treatment baths.

Comparing the results of Example 2 and Comparison 2 to Comparison 3 withrespect to glass voids (GV) shows that the presence of the homopolymerpolyelectrolyte dissolved in the neutralizer solution improves glassconditioning (i.e., eliminating all glass voids in both double-sided andmultilayer PWB in Example 2 and Comparison 2).

While the invention has been described above with reference to specificembodiments thereof, it is apparent that many changes, modifications,and variations can be made without departing from the inventive conceptdisclosed herein. Accordingly, it is intended to embrace all suchchanges, modifications, and variations that fall within the spirit andbroad scope of the appended claims. All patent applications, patents,and other publications cited herein are incorporated by reference intheir entirety.

What is claimed is:
 1. The process for electroplating a metal layer tothe surface of a nonconductive material which comprised the steps:(a)contacting said substrate surface with an alkaline permanganate solutionfor an effective time and at an effective concentration and at anelevated temperature to prepare said substrate surface for a metal layerto be electroplated thereto; (b) then contacting said substrate surfacewith an aqueous neutralizer/conditioner solution, said solutioncomprising water, at least one neutral or acidic reducing agent, and atleast one polyelectrolyte polymer conditioner; (c) then contacting saidsubstrate surface with a liquid dispersion of carbon black comprisedof:(1) carbon black particles having an average particle size of lessthan about 3 microns in said dispersion; (2) an effective dispersingamount of a surfactant which is compatible with said carbon black; and(3) a liquid dispersing medium, wherein the amount of carbon black issufficient to coat substantially all of the nonconducting substratesurface and is less than about 4% by weight of said liquid dispersion;(d) separating substantially all of the liquid dispersing medium fromsaid applied dispersion, thereby depositing said carbon black particlesin a substantially continuous layer on said nonconductive substratesurface; and (e) electroplating a substantially continuous metal layerover the deposited carbon black layer on said nonconducting substratesurface.
 2. The process of claim 1, wherein said alkaline permanganateis made from a mixture comprising water, an alkaline hydroxide, analkaline permanganate, and optionally, a secondary oxidizer selectedfrom the group consisting of an alkaline hypochlorite or an alkalinepersulfate.
 3. The process of claim 1, wherein said permanganatecontacting step (a) is preceded by a solvent swell treatment.
 4. Theprocess of claim 1 wherein said reducing agent employed in step (b) isselected from the group consisting of dihydrazine sulfate andhydroxylamine salts.
 5. The process of claim 1, wherein saidpolyelectrolyte polymer conditioners employed in step (b) is selectedfrom the group consisting of cationic polyelectrolyte, polyacrylamidehomopolymer resins and cationic polyelectrolyte polyamine homopolymerresins.
 6. The process of claim 1, wherein said liquid carbon blackdispersion further comprises a sufficient amount of at least onealkaline hydroxide to raise the pH of said liquid dispersion in therange from about 10 to
 14. 7. The process of claim 1, wherein saidliquid dispersion contains less than about 10% by weight solidsconstituents.
 8. The process of claim 1 wherein said liquid dispersingmedium is water.
 9. The process of claim 1, wherein said conductivemetal is copper.
 10. The process for electroplating the walls of throughholes in a laminated printed wiring board comprised of at least onenonconducting layer laminated to and alternating with at least twoseparate conductive metal layers, which comprises the steps:(a)contacting said substrate having said through holes with an alkalinepermanganate solution for an effective time and at an effectiveconcentration and at an elevated temperature to desmear the walls ofsaid through holes; (b) then contacting said substrate having saidthrough holes with a neutralizer/conditioner solution, solutioncomprising a mixture of water, at least one neutral or acidic reducingagent, and at least one polyelectrolyte polymer conditioner; (c) thencontacting said substrate having said through holes with a liquiddispersion of carbon black comprised of:(1) carbon black particleshaving an average particle size of less than about 3.0 microns in saiddispersion; (2) an effective dispersing amount of a surfactant which iscompatible with said carbon black; and (3) a liquid dispersing medium,wherein the amount of carbon black is sufficient to coat substantiallyall of said nonconducting surfaces of the hole walls and is less thanabout 4% by weight of said liquid dispersion; (d) separatingsubstantially all of the liquid dispersing medium from said applieddispersion, thereby depositing said carbon black particles in asubstantially continuous layer on said nonconducting portions of saidhole walls; (e) microetching said metal layers of said printed wiringboard to remove deposited carbon black therefrom; and (f) laterelectroplating a substantially continuous metal layer over the depositedcarbon black layer on said nonconducting portions of hole walls, therebyelectrically connecting said metal layers of said printed wiring board.11. The process of claim 10 wherein said liquid carbon black dispersionfurther comprises a sufficient amount of at least one alkaline hydroxideto raise the pH of said liquid dispersion in the range from about 10 to14.
 12. The process of claim 11 wherein said alkaline hydroxide ispotassium hydroxide.
 13. The process of claim 12 wherein said liquiddispersion contains less than about 10% by weight solids constituents.14. The process of claim 13 wherein said carbon black particles have aninitial pH from about 2 to
 4. 15. The process of claim 14 wherein saidsurfactant is a phosphate ester anionic surfactant.
 16. The process ofclaim 10 wherein said conductive metal is copper.
 17. The process ofclaim 10 wherein said liquid dispersing medium is water.
 18. The processof claim 10 wherein said microetch in step (c) comprises sodiumpersulfate and sulfuric acid.